This invention relates to circuits and methods for measuring electrical energy, and more particularly to electrical energy measurement circuits and methods involving current sensors of the type which generate a signal proportional to the derivative of a current with respect to time (di/dt).
Meters for measuring electrical energy conventionally measure current and voltage on the metered power line and multiply the measured current and voltage to produce a signal representative of the electrical power consumed by a load connected to the metered power line. One known way of measuring electrical current is with an inductive current pickup, which produces a signal proportional to di/dt. Current sensors of this type have been referred to as mutual inductance transformers, mutual inductance current transducers, and embedded coils, and are described, for example, in the following patents:
______________________________________ Patent No. Inventor Issue Date ______________________________________ 3,226,641 Miller Dec. 28, 1965 4,182,983 Heinrich et al. Jan. 8, 1980 4,250,552 Elms Feb. 10, 1981 4,368,424 Miller Jan. 11, 1983 4,473,810 Souques et al. Sep. 25, 1984 4,591,810 Mackenzie et al. May 27, 1986 4,596,951 Heinrich et al. Jun. 24, 1986 5,053,697 Carnel et al. Oct. 1, 1991 ______________________________________
An improved design of an embedded coil is also disclosed in co-pending U.S. patent application Ser. No. 07/833,738, assigned to the assignee of the present invention, which application is hereby incorporated by reference.
Because the output of the embedded coil or other inductive current pickup is a voltage proportional to the derivative of the load current, the current sensor output must be integrated to obtain a signal proportional to the load current. This general concept is illustrated in FIG. 1, and various forms of integrators are disclosed in the above-referenced patents.
Ideally, an inductive current pickup exhibits a phase shift of precisely 90.degree., which can be offset by a simple integrator circuit consisting of a capacitor in the feedback loop of an operational amplifier (op amp). A basic integrator of this type is shown in the Mackenzie patent referenced above, in which a second integrator having its input connected to a square-wave clock signal is also employed, with the outputs of the two integrators being added together and supplied to a comparator which generates a pulse-width-modulated (PWM) signal having a duty cycle proportional to metered current. A negative feedback path including a low-pass filter is connected between the comparator output and the input to the basic integrator to compensate for errors produced by the integrator and comparator, thus, according to the patent, permitting the use of low-cost inverter circuitry fabricated on an integrated circuit. A similar configuration is disclosed in U.S. Pat. No. 4,596,951 to Heinrich et al., in which one form of the pulse-width modulator includes a capacitor and a plurality of resistive components in the feedback network of an op amp, along with a bypass capacitor connected to the negative regulated reference voltage supply and one of the resistive components in the feedback network.
An integrator with a parallel resistor-capacitor (RC) network in the feedback loop of an op amp connected to a di/dt sensor is disclosed in U.S. Pat. No. 4,182,983 to Heinrich et al., which also relates to a PWM technique.
A similar integrator circuit is shown in U.S. Pat. No. 4,250,552 to Elms, in which the output of the integrator is connected to a twin-T network.
U.S. Pat. No. 5,053,697 to Carnel et al. discloses an active filter connected as an op amp integrator having a feedback capacitor, one end of which is connected to the op amp inverting input, a coupling capacitor connected between the op amp output and the other end of the feedback capacitor, and a gain-adjusting resistor directly connecting the op amp output to its inverting input. The circuit also includes a passive RC filter of the integrator type connected upstream from the two inputs to the op amp, for the stated purpose of compensating for a difference between the phase shift imparted by the active filter and a predetermined value desired for said phase shift.
U.S. Pat. No. 3,226,641 to Miller shows a bridged-T network in the feedback path of an amplifier, the bridge-T network including a capacitor and a T network consisting of two resistors in series, with the common leads of the resistors connected to ground through a second capacitor. A third capacitor is connected in parallel with an input resistor to compensate for secondary inductance of the current transformer.
The actual phase shift exhibited by an inductive current pickup is not precisely 90.degree. due to eddy currents set up in the conductors exposed to the magnetic field and eddy currents in the magnetic material in the flux path. Therefore, an integrator exhibiting a phase shift of precisely 90.degree. is not necessarily the solution to the problem of converting the inductive pickup output signal to a signal proportional to current. Although a desired small deviation from 90.degree. phase shift in the integrator can be achieved by setting the corner frequency of a first-order low-pass filter to a sufficiently low frequency, e.g., under 1 Hz for a desired phase deviation on the order of 0.5.degree., it has been found that some undesirable trade-of is usually required, either in the form of a resistance value which is excessively high for a desired specification of DC offset, or because the required capacitor is either too large in size, too sensitive to temperature, or more expensive than desired in the increasingly cost-sensitive market for watthour meters and the like.
Thus, there remains a need, in electrical energy meters of the type having a di/dt current sensor, for an improved integrator design offering stability with time and temperature, linear performance, low DC output voltage, low sensitivity to component tolerances and frequency variations, low component count, and compensation for nonideal phase characteristics of embedded coils and the like.